This invention relates to a brake fluid pressure control device which can perform both antilock brake control and automatic brake control such as traction control. The device is basically an antilock brake control system with extra elements added to perform automatic braking control.
Currently, most widely used antilock fluid pressure control devices are those including an electromagnetically driven wheel brake fluid pressure control valve with a discharge valve provided in a main fluid line connecting the master cylinder to each wheel brake, a fluid reservoir for temporarily storing brake fluid discharged through the discharge valve, and a motor-driven pump for sucking and returning the fluid in the reservoir into the main fluid line (hereinafter referred to as fluid-return type antilock control device), because they are economical.
The most economical way to add automatic brake control functions such as traction control function to the fluid-return type antilock control device would be to use the pump for antilock fluid pressure control as a fluid pressure source for automatic brake control by modifying the brake line so that brake fluid can be supplied from the pump to each wheel brake. Devices of this type are already known.
For example, examined Japanese patent publication 5-65388 discloses a device having a valve for connecting a fluid supply circuit to the main circuit only during traction control and otherwise disconnect from the main circuit.
Unexamined Japanese patent publication 4-231241 discloses a device having a second reservoir having a driving means and used as an active pressure accumulator.
Unexamined Japanese patent publication 5-116607 discloses a device having a second reservoir which, having no driving means, is used as a passive pressure accumulator.
If the pump for antilock fluid pressure control is used as a fluid pressure source during automatic brake control, the pressure rising speed will drop, deteriorating the responsiveness of the device, or otherwise an extremely large pump-driving motor has to be used.
This is because the wheel brake is low in rigidity while the pressure is low and due to the output characteristics of the motor. The revolving speed of the motor increases linearly with an increase in the torque loaded. In the low-load region, its output is low. The characteristics of the motor and the pump should be such that a maximum pressure is produced when the motor output is maximum. But in this case, the amount of fluid discharged from the pump in the low-load region will be only twice the amount of fluid discharged when the maximum pressure is produced, due to the characteristics of the motor. Since the wheel brake is low in rigidity in the low-pressure region, a long time is taken in this region. The pressure rising speed may be thus insufficient according to the object of automatic braking. It is of course possible to increase the pressure rising speed by using a large-output motor. But even such a motor can produce only an output that is far smaller than its maximum output in the low-pressure region. In the high-pressure region, the output tends to be rather excessive, which is a waste of energy. That is, using a large-output motor will merely result in increases in size and cost.
An object of this invention is to provide a device which can increase the pressure at a higher rate to a higher level without using a large motor while keeping high efficiency in the low-load region by adding a minimum number of elements.